Add host aliasing to service discovery

README.md

@@ -1,4 +1,4 @@
-# Terraform svchost package
+# terraform-svchost
 
 [![CI Tests](https://github.com/hashicorp/terraform-svchost/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/hashicorp/terraform-svchost/actions/workflows/ci.yml)
 [![GitHub license](https://img.shields.io/github/license/hashicorp/terraform-svchost.svg)](https://github.com/hashicorp/terraform-svchost/blob/main/LICENSE)

disco/disco.go

@@ -10,14 +10,13 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"mime"
 	"net/http"
 	"net/url"
 	"time"
 
-	"github.com/hashicorp/terraform-svchost"
+	svchost "github.com/hashicorp/terraform-svchost"
 	"github.com/hashicorp/terraform-svchost/auth"
 )
 
@@ -42,6 +41,7 @@ var httpTransport = defaultHttpTransport()
 // hostnames and caches the results by hostname to avoid repeated requests
 // for the same information.
 type Disco struct {
+	aliases   map[svchost.Hostname]svchost.Hostname
 	hostCache map[svchost.Hostname]*Host
 	credsSrc  auth.CredentialsSource
 
@@ -69,6 +69,7 @@ func New() *Disco {
 // the given credentials source.
 func NewWithCredentialsSource(credsSrc auth.CredentialsSource) *Disco {
 	return &Disco{
+		aliases:   make(map[svchost.Hostname]svchost.Hostname),
 		hostCache: make(map[svchost.Hostname]*Host),
 		credsSrc:  credsSrc,
 		Transport: httpTransport,
@@ -104,11 +105,15 @@ func (d *Disco) CredentialsSource() auth.CredentialsSource {
 }
 
 // CredentialsForHost returns a non-nil HostCredentials if the embedded source has
-// credentials available for the host, and a nil HostCredentials if it does not.
+// credentials available for the host, or host alias, and a nil HostCredentials if it does not.
 func (d *Disco) CredentialsForHost(hostname svchost.Hostname) (auth.HostCredentials, error) {
 	if d.credsSrc == nil {
 		return nil, nil
 	}
+	if aliasedHost, aliasExists := d.aliases[hostname]; aliasExists {
+		log.Printf("[DEBUG] CredentialsForHost found alias %s for %s", hostname, aliasedHost)
+		hostname = aliasedHost
+	}
 	return d.credsSrc.ForHost(hostname)
 }
 
@@ -139,6 +144,13 @@ func (d *Disco) ForceHostServices(hostname svchost.Hostname, services map[string
 	}
 }
 
+// Alias accepts an alias and target Hostname. When service discovery is performed
+// or credentials are requested for the alias hostname, the target will be consulted instead.
+func (d *Disco) Alias(alias, target svchost.Hostname) {
+	log.Printf("[DEBUG] Service discovery for %s aliased as %s", target, alias)
+	d.aliases[alias] = target
+}
+
 // Discover runs the discovery protocol against the given hostname (which must
 // already have been validated and prepared with svchost.ForComparison) and
 // returns an object describing the services available at that host.
@@ -176,6 +188,11 @@ func (d *Disco) DiscoverServiceURL(hostname svchost.Hostname, serviceID string)
 // discover implements the actual discovery process, with its result cached
 // by the public-facing Discover method.
 func (d *Disco) discover(hostname svchost.Hostname) (*Host, error) {
+	if aliasedHost, aliasExists := d.aliases[hostname]; aliasExists {
+		log.Printf("[DEBUG] Discover found alias %s for %s", hostname, aliasedHost)
+		hostname = aliasedHost
+	}
+
 	discoURL := &url.URL{
 		Scheme: "https",
 		Host:   hostname.String(),
@@ -259,7 +276,7 @@ func (d *Disco) discover(hostname svchost.Hostname) (*Host, error) {
 	// size, but we'll at least prevent reading the entire thing into memory.
 	lr := io.LimitReader(resp.Body, maxDiscoDocBytes)
 
-	servicesBytes, err := ioutil.ReadAll(lr)
+	servicesBytes, err := io.ReadAll(lr)
 	if err != nil {
 		return nil, fmt.Errorf("error reading discovery document body: %v", err)
 	}
@@ -284,3 +301,10 @@ func (d *Disco) Forget(hostname svchost.Hostname) {
 func (d *Disco) ForgetAll() {
 	d.hostCache = make(map[svchost.Hostname]*Host)
 }
+
+// ForgetAlias removes a previously aliased hostname as well as its cached entry, if any exist.
+// If the alias has no target then this is a no-op.
+func (d *Disco) ForgetAlias(alias svchost.Hostname) {
+	delete(d.aliases, alias)
+	d.Forget(alias)
+}

disco/disco_test.go

@@ -10,7 +10,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/hashicorp/terraform-svchost"
+	svchost "github.com/hashicorp/terraform-svchost"
 	"github.com/hashicorp/terraform-svchost/auth"
 )
 
@@ -362,6 +362,74 @@ func TestDiscover(t *testing.T) {
 		}
 
 	})
+
+	t.Run("alias", func(t *testing.T) {
+		// The server will listen on localhost and we will expect this response
+		// by requesting discovery on the alias.
+		portStr, close := testServer(func(w http.ResponseWriter, r *http.Request) {
+			resp := []byte(`
+{
+"thingy.v1": "http://example.com/foo"
+}
+`)
+			w.Header().Add("Content-Type", "application/json")
+			w.Header().Add("Content-Length", strconv.Itoa(len(resp)))
+			w.Write(resp)
+		})
+		defer close()
+
+		target, err := svchost.ForComparison("localhost" + portStr)
+		if err != nil {
+			t.Fatalf("test server hostname is invalid: %s", err)
+		}
+		alias, err := svchost.ForComparison("not-a-real-host-dont-even-try.no")
+		if err != nil {
+			t.Fatalf("alias hostname is invalid: %s", err)
+		}
+
+		d := New()
+		d.SetCredentialsSource(auth.StaticCredentialsSource(map[svchost.Hostname]map[string]any{
+			target: {
+				"token": "hunter2",
+			},
+		}))
+
+		d.Alias(alias, target)
+
+		discovered, err := d.Discover(alias)
+		if err != nil {
+			t.Fatalf("unexpected discovery error: %s", err)
+		}
+
+		gotURL, err := discovered.ServiceURL("thingy.v1")
+		if err != nil {
+			t.Fatalf("unexpected service URL error: %s", err)
+		}
+		if gotURL == nil {
+			t.Fatalf("found no URL for thingy.v1")
+		}
+		if got, want := gotURL.String(), "http://example.com/foo"; got != want {
+			t.Fatalf("wrong result %q; want %q", got, want)
+		}
+
+		aliasCreds, err := d.CredentialsForHost(alias)
+		if err != nil {
+			t.Fatalf("unexpected credentials error: %s", err)
+		}
+		if aliasCreds.Token() != "hunter2" {
+			t.Fatalf("found no credentials for alias")
+		}
+
+		d.ForgetAlias(alias)
+
+		discovered, err = d.Discover(alias)
+		if err == nil {
+			t.Error("expected error, got none")
+		}
+		if discovered != nil {
+			t.Error("expected discovered to be nil, got non-nil")
+		}
+	})
 }
 
 func testServer(h func(w http.ResponseWriter, r *http.Request)) (portStr string, close func()) {